i have had the sa6 and now the custom ct6 from sleek. they are worlds apart. the same must be true of the um3x and the es3x. the reason basically is bone conduction. with bone conduction. you don't only hear the sounds, but your ourter and inner ear feel the vibrations which are not dampened at all as with flanges and foams and even the custom ear pieces that you an build.
the tube is bigger and that probably makes a difference, but it also means that that big tube feeds more vibrations into the places that matter most. trust me, my sleek ct6 is leagues better than the se530 which costs more. if the se530 was housed in a nice custom, it would probably be a completely different animal.
And the above shows one of the problems I've also mentioned before - our concept/ definition of good/ better/ worlds apart/huge/ significant/ etc. -- a problem I also notice when speaking/reading about portable amps (AND LOD's), though I've to admit my experience there is much more limited to the RSA P-51 Mustang & Nuforce Icon Mobile and 3 LOD's ranging from $30-$80.